1. Field of the Invention
This invention relates to a noise generator, and more particularly to a noise generator which is capable of producing from one noise generating circuit a noise having a desired harmonic composition and a band noise set at a desired frequency with high accuracy.
2. Description of the Prior Art
As a noise source for use as a rhythm source of an electronic musical instrument, use has heretofore been made of a pseudorandom pulse generator shown in FIG. 1.
In the pseudo-random pulse generator of FIG. 1, for example, outputs from first and fifteenth stages of a 15-stage shift register 1 driven by a clock generator 5 are applied to an exclusive OR circuit 2 to derive therefrom "1" or "0" in dependence on whether the two inputs are of different levels or of the same level, and the output from the exclusive OR circuit 2 is returned via an OR circuit 3 to the T input of the shift register 1 to provide 2.sup.15 -1(32767) pseudo-random pulses from its fifteenth stage output Q.sub.15.
The output from a NOR circuit 14 supplied with each stage output from the shift register 1 is applied via the OR circuit 3 to the T input of the shift register 1 to remove the state of "0" at its all stages, providing the initial condition that "1" is produced at all the stages.
By taking out parallel outputs Q.sub.1 to Q.sub.15 from the respective stages of the shift register 1 using the pseudo-random pulse generator 10 of such a construction, a successively changing random pulse output is obtained. The random pulse output is applied to a D-A converter 6 for conversion into an analog signal, which is provided via a low-pass filter 7 to a sound system 8.
In this case, unless a variable resistance of the clock generator 5 is varied, the harmonic composition of the noise derived from the low-pass filter 7 remains unchanged, and in the case of requiring two or more noise sources, it is necessary to prepare pseudo-random pulse generators of the same number as the required noise sources. In such a case, since a band noise is limited by an ordinary analog filter, highly precise frequency setting is difficult. In recent years, however, it is sometimes necessary to set frequencies with high accuracy, as rhythm sources of a cymbal and so forth, corresponding not only to integer order harmonics of a fumdamental frequency but also to its non-integer (fractional) order harmonics.